1. Field of the Invention
The present invention relates to an apparatus and method for evaluating the shape of a three-dimensional object, and particularly, to such an apparatus and method that evaluate the precision at a worked surface of a three-dimensional object by comparing the worked surface of the three-dimensional object actually measured in the form of space coordinates at a plurality of measurement points with an ideal surface of the three-dimensional object which has been stored in the form of three-dimensional curved surface data.
2. Description of the Prior Art
There is known a three coordinate measuring machine which measures the shape of a surface, which includes a curved surface, of a three-dimensional object. Such a three-coordinate measuring machine determines space coordinates at a plurality of measurement points by contacting a probe on the surface of the object and grasps the surface of the object based on a set of space coordinates. The three coordinate measuring machine is utilized in evaluating the precision of the three-dimensional shape of a workpiece. The surface of the workpiece measured by the three coordinate measuring machine is compared with an ideal surface of the workpiece defined by CAD shape data or design drawing data. The difference between the measured and the ideal surfaces determines the precision of the surface of the workpiece. When the workpiece is to be finished while utilizing the measurement of the three coordinate measuring machine, the amount of material cut off from the workpiece, that is, the distance from the measured surface of the workpiece to the ideal surface of the same (distance perpendicular to the ideal surface) must be determined.
For example, Japanese Patent Publication No. Hei 1-26817 discloses a system for determining such a perpendicular distance. Such a system first determines a reference point of a space coordinate on the ideal surface defined by the CAD shape data and also a perpendicular vector at the reference point. By using the perpendicular vector, the actual space coordinates of the worked surface of the three-dimensional object are then measured through the three coordinate measuring machine in the direction perpendicular to the ideal surface. The distance between the measured space coordinates and the reference point is determined by the perpendicular vector. In this manner, the perpendicular distance from the measured worked surface to the ideal surface based on the CAD shape data can be determined. In this example, the above results are utilized to display the surface of the workpiece as an image similar to contour lines, in which the sections are painted with different colors in accordance with the magnitude of the distance. The determined distance is also used to control marking on a workpiece.
In the aforementioned system of the prior art, the perpendicular vector must previously be determined from the CAD shape data and the probe should be moved along the perpendicular vector to determine the space coordinates of the actual surface. Such a perpendicular measurement cannot be carried out by the conventional three coordinate measuring machine, which requires an additional control mechanism for moving the probe in the direction perpendicular to the ideal surface. The three coordinate measuring machine for such a measurement raises another problem in that it must work upon receipt of the CAD shape data, so that the machine cannot be applied to systems other than a certain system which is connected to and controlled on-line by a CAD system. It is still difficult to move the probe precisely in the perpendicular direction, leading to an error upon measurement of the distance.